Hawking's betlost in 1990he had bet it was not a black hole[1]
About the image. Drawn as a blue supergiant with a stream of gas flowing into a small bright disk around the black hole. The pair, their colours and their orbit are measured; the shape of the gas stream is modelled.
In 1964 an Aerobee sounding rocket launched from White Sands, New Mexico, carried X-ray detectors above the atmosphere for a few minutes and found a strong source in the constellation Cygnus. It became Cygnus X-1. When the Uhuru X-ray satellite looked at it in the early 1970s, the source flickered several times a second, which meant it could be no more than about 100 000 kilometres across.
Weighing an invisible companion
In 1971 radio observations pinned the source to a blue supergiant star, HDE 226868. The next year Louise Webster and Paul Murdin, and independently Tom Bolton, found that the star swings back and forth every 5.6 days around a companion that could not be seen, yet was too heavy to be a neutron star. By the end of 1973 most astronomers agreed the companion was a black hole. Cygnus X-1 was the first X-ray source widely accepted as one.
The supergiant, about 29 times the Sun’s mass, is losing gas to its partner. The gas spirals into a hot disk around the black hole and shines brightly in X-rays; Cygnus X-1 is usually the brightest persistent source of hard X-rays in the sky. The black hole also drives jets carrying more than a thousand times the Sun’s power.
Hawking’s insurance policy
In December 1974 Stephen Hawking made a bet with Kip Thorne that Cygnus X-1 was not a black hole. He called it an insurance policy: he had invested so much work in black holes that if they turned out not to exist, at least he would win the bet. The prize for Hawking was four years of the magazine Private Eye. In 1990, with the evidence overwhelming, he conceded.
How heavy?
A 2021 study by James Miller-Jones and colleagues put the black hole at about 21 solar masses. A 2025 analysis revised it down to roughly 14 to 18. Either way, its event horizon is tiny: about 44 kilometres in radius for 15 solar masses, about 62 for 21.
To scale · radius, Sun = 1Sun: 1.00 · Cygnus X-1 event horizon: 0.000Event horizon about 44 km; the Sun's radius is 695 700 km. At this scale the black hole is far too small to see.
How long to get there?
Cygnus X-1 is about 7 200 light-years away.
Light299 792 km/s7 200 years
Breakthrough Starshotplanned light sail, 20% of light speed36 000 years
Parker Solar Probefastest spacecraft so far, 192 km/s11.2 million years
Voyager 1leaving the Solar System, 17 km/s127.0 million years
At Voyager 1's speed the trip would take about 1 587 134 human lifetimes of 80 years. Bars are on a logarithmic scale. Speeds: NASA, Breakthrough Initiatives.
Gallery
A large blue star with a thin stream of gas flowing from it to a small black hole ringed in orange, an impression of Cygnus X-1. AI-generated painting.A black hole surrounded by a swirling white and orange disc of gas, with the blue glow of a companion star at the top edge, an impression of Cygnus X-1. AI-generated painting.Cygnus X-1, 3D artist's impression, rendered from the interactive model.Cygnus X-1, 3D artist's impression, from another angle, rendered from the interactive model.Cygnus X-1, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
1964Found by X-ray detectors on a sounding rocket.
1971Pinned to the blue supergiant HDE 226868.
1972An unseen, heavy companion is found.
1974Stephen Hawking bets Kip Thorne it is not a black hole.
1990Hawking concedes the bet.
2021Its mass is measured at about 21 Suns; revised lower in 2025.
Questions people ask
Was Cygnus X-1 the first black hole discovered?
In effect, yes. It was the first X-ray source widely accepted as a black hole, by the end of 1973.
What did Stephen Hawking bet?
That Cygnus X-1 was not a black hole. He conceded in 1990 and owed Kip Thorne a magazine subscription.
How heavy is Cygnus X-1?
Between about 14 and 21 times the Sun, depending on the study.